[gdal-dev] GeoTiff with ColorInterp=Palette

[Even Rouault]

Le 20/02/2024 à 11:15, Elena Ruiz via gdal-dev a écrit :
> Hello, I have version 3.6.2 of GDAL and I need to crop an image using 
> a contour from a GML file, so far there is no problem.
>
> The problem begins when the original image is a GEOTIFF with 
> transparency and it generates a cropped image, the information of both 
> images is a little lower.
>
> The color table changes from having 2 values to having 256, this 
> causes it to lose transparency although it has the label "Mask Flags: 
> PER_DATASET ALPHA" which indicates that it has an alpha channel.
>
> My question is, why does the color table change?, and is there any 
> parameter that can be used with gdalwarp or gdal_translate to avoid 
> this and keep the image transparent?

Strictly speaking your input image has no transparency, just black & 
white colors. Not sure which one you interpret as transparent?


You could force a 1-bit output image (ie with a color palette of 2 
colors) by adding -co NBITS=1 . But that will likely won't play well 
with -dstalpha.  You'd then want to use instead -dstnodata 0 if black is 
the transparent color or -dstnodata 1 if white is the transparent color


Even

-- 
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